Device for supplying electricity to a motor vehicle

ABSTRACT

A system for supplying electricity to a motor vehicle using a capacitor having a maximum value greater than the maximum value of a rechargeable battery. A transformer provides discharging of the capacitor from its maximum voltage down to the maximum voltage of the battery in order to provide excess energy in a short period of time to effectively charge the battery and simultaneously increase the service life of the chargeable battery.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German Application 19754964.0,filed Dec. 11, 1997, the disclosures of which is expressly incorporatedby reference herein.

The invention relates to a device for supplying electricity to a motorvehicle according to the preamble of claim 1.

A device of this type is known, for example, from German Patent DocumentDE 43 40 350 C2. This known device has a rechargeable battery, acapacitor which can be switched in parallel to the battery, and acircuit arrangement in the form of a logic circuit arranged between thebattery and the capacitor. When the starter in the vehicle is actuated,the logic circuit defines a time window and carries out at least onevoltage query. As a function of the result of this voltage query, thelogic circuit switches the capacitor in parallel to the battery. Thecapacitor is preferably switched in parallel only when the voltage ofthe battery within the time window falls below a predetermined value. Bymeans of this known circuit arrangement, with the capacitor switched inparallel to the battery, the starting of the vehicle is still ensuredwhen the battery is almost discharged. However, with the parallelconnection of the capacitor with the battery, the maximum capacitorvoltage cannot exceed the value of the battery voltage. This knowncircuit arrangement is therefore not suitable for storing in thecapacitor a high excess of energy for a short time and thus foreffectively recharging a discharged battery.

In addition, a circuit arrangement for supplying electricity to a motorvehicle is known from German Patent Document DE 195 22 563 A1, in thecase of which the energy stored in a capacitor, particularly theelectric energy generated during a recuperative braking, is fed to arechargeable battery in a controlled manner. However, this known circuitarrangement discloses no details concerning the type of the control orthe ratio of the maximally possible capacitor voltage to the batteryvoltage.

Furthermore, with respect to the technical environment, reference ismade to European Patent Document EP 0 568 655 B1, from which a device isknown for supplying electricity to a motor vehicle which has twochargeable batteries of different nominal voltages and a circuitarrangement in the form of a voltage transformer arranged between thebatteries.

It is an object of the invention to provide an improved device of typefor supplying of electricity to a motor vehicle such that, on the onehand, an energy excess which is available for a short time iseffectively utilized for charging a vehicle battery and simultaneouslythe service life of the rechargeable vehicle battery is increased.

The use of a capacitor (such as a Power Cap or Super Cap) whose nominalvoltage and thus its maximally possible voltage is preferably severaltimes higher than the nominal voltage of the battery is essential to theinvention. By using such a buffer capacitor with a large voltagevariation range as the energy accumulator, the voltage at the capacitorcan be significantly increased beyond the battery voltage in order to beable to store in the best possible manner, a short-term energy excess,as, for example, resulting from recuperative braking. The battery ischarged in a controlled manner by means of this capacitor by using avoltage transformer, preferably a DC/DC converter.

In a particularly advantageous further development of the invention, thecharging of the battery is controlled by means of the capacitor by wayof the voltage converter such that the charged capacitor is maximallydischarged until a capacitor voltage is reached which is approximatelyequal to the momentary actual voltage of the battery. By means of thisadvantageous further development, a circuit arrangement can be used as avoltage transformer which only has to carry out a “downward”transformation in the sense of a voltage reduction starting from thecapacitor voltage. As the result, the voltage transformer can be built,in a particularly simple manner, at reasonable cost, between the batteryand the capacitor.

The invention also includes an expanded voltage transformer in such amanner that, in the reverse direction, the capacitor can be charged byway of the battery to a voltage whose value is larger than the value ofthe battery voltage.

The circuit arrangement according to the invention is used in the caseof motor vehicles with a chargeable battery which has a higher thanconventional nominal voltage (for example, 36 V instead of 12 V) inorder to ensure the supply of high-power consuming devices whose numberis constantly increasing in motor vehicles.

With a device for supplying electricity according to the invention, onthe one hand, an energy excess which is available for a short time iseffectively utilized and, on the other hand, a variable multivoltageelectrical wiring is permitted.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing illustrates an embodiment of the invention.

FIG. 1 is a view of a circuit arrangement according to the invention;

FIG. 2 is a view of a possible course of the capacitor voltage accordingto the control of the invention for charging the battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, a capacitor 1 is connected by way of a voltage transformer 2,which preferably is a DC/DC converter, with a vehicle battery 3. As thecapacitor 1, a buffer capacitor (power Cap), is preferably used whichhas a nominal voltage or maximally possible voltage U_(C max) of, forexample, 80 V. The battery 3 is, for example, a conventional batterywith a nominal voltage U_(B) of, for example, 36 V. Thus, the nominalvoltage of the capacitor 1 is larger than the nominal voltage of thebattery approximately by a factor 2.

The capacitor 1 can be charged by way of an electric connection A whichis connected, for example, with a generator for the braking energyrecirculation. The capacitor voltage U_(C) is directly proportional tothe charging condition of the capacitor 1. The ratio of the chargingcondition or of the charged amount of energy E to the capacitor voltageU_(C) is obtained by the following formula: E=½·C·U_(C) ². In the caseof the double voltage U_(C), four times the amount of Energy E cantherefore be accumulated.

Additionally, as a result of its cycle stability and full-loadstability, the service life of such a capacitor is significantly longerthan that of a conventional motor vehicle battery.

If the capacitor 1 is charged at least in such a manner that thecapacitor voltage U_(C) is higher than the actual voltage U_(B actual)of the battery 3, the voltage transformer 2 controls the charging of thebattery 3 by the capacitor 1 corresponding to the requirement of thebattery 3 and/or an electrical wiring (not shown here) optionallyconnected with the battery 3.

FIG. 2 illustrates in detail the manner and mode of the control of thevoltage transformer 2. On the X-axis, FIG. 2 shows the load condition Lor the accumulated amount of energy E and, on the Y-axis, FIG. 2 showsthe pertaining capacitor voltage U_(C). According to the invention, forexample, based on a completely charged capacitor 1 (U_(C)=U_(C max)),the charging of the battery 3 by the discharging of the capacitor 1 ismaximally carried out until the capacitor voltage U_(C) hasapproximately reached the value of the actual voltage U_(B actual) ofthe battery voltage 3. As a result, the voltage transformer 2 must carryout only a voltage downward transformation. If the capacitor 1 were tobe discharged further, starting from the falling below the capacitorvoltage U_(C)=U_(B actual), the voltage transformer 2 would have tocarry out a voltage upward transformation in the sense of a voltageincrease. Although this can be technically implemented, it isinefficient in view of the requirement to have an efficiency which is ashigh as possible.

If, as in the illustrated embodiment according to FIG. 2, a discharge ofthe capacitor 1 has taken place starting from a voltage U_(C)=80V insuch manner that the capacitor voltage U_(C) has reached the value ofthe actual voltage U_(B actual)=36 V (equal to the nominal voltageU_(B)=36 V) of the battery 3, because of the above-mentionedrelationships between the load condition L or the amount of energy E andthe capacitor voltage U_(C), in the event of a decrease of the capacitorvoltage U_(C) by approximately half, ¾ of the amount of energy Eaccumulated in the capacitor was already delivered to the battery 3.

By means of this control of the charging of the battery according to theinvention, an optimal compromise is achieved between the circuit-relatedexpenditures of the voltage transformer 2 and a utilization of theenergy excess charged in the capacitor 1 for a short time, which is asefficient as possible.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A device for supplying electricity to a motorvehicle, comprising: a chargeable battery; a voltage transformer controldevice having a first end connected to said chargeable battery; acapacitor for charging said chargeable battery connected to a second endof said voltage transformer control device wherein the maximum voltageof said capacitor is greater than a maximum voltage of said battery andwherein said transformer control device discharges said capacitor from atime that a voltage of said capacitor reaches said maximum voltage ofsaid capacitor until said voltage of said capacitor is substantiallyequal to said maximum voltage of said battery.
 2. A device according toclaim 1, wherein said capacitor is discharged until the voltage of saidcapacitor is equal to the value of the actual voltage of the battery. 3.A method for supplying electricity to a motor vehicle, comprised thesteps of: providing a rechargeable battery having a nominal voltage;providing an energy accumulator having a maximum voltage which issubstantially greater than said nominal voltage; discharging said energyaccumulator from a time that a voltage on said accumulator reaches saidmaximum voltage of said accumulator until the voltage of saidaccumulator is substantially equal to said nominal voltage of saidrechargeable battery.